This application claims the priority of German Application 19916806.7, filed Apr. 14, 1999, the disclosure of which is expressly incorporated by reference herein.
The invention relates to an ignition device for piston engines with a moveable piston electrode in the piston face and a counter-electrode in the cylinder head with a spark gap between them.
An ignition device is disclosed in WO 96/13660 A. It is a xe2x80x9cspark to pistonxe2x80x9d device in which the piston electrode protrudes slightly from the piston floor and is flush with a counter-electrode approximately in the center of the combustion chamber. Such a device is not suitable especially for igniting extremely lean mixtures, since the spark gap has a minimal length only at the top dead center of the piston. In that case whether or not an ignition spark actually forms depends on various factors which differ over the entire range of operation of - the internal combustion engine. These factors include a sufficiently ignitable mixture, the working temperature, the ignition energy and the steepness of the flanks for the build-up of the secondary voltage.
The invention is addressed to the problem of creating an ignition device which will securely initiate the ignition even of extremely lean mixtures.
In accordance with the present invention, due to the short spark length in comparison to the distance of the counter-electrode from the piston bottom, the ignition energy needed for the ignition of the fuel-air mixture is substantially lower than in prior art systems. Therefore the ignition reliability is increased for the same ignition energy.
In one of the structural embodiments of the present invention, the length of the spark gap is kept approximately constant for a period that is longer than the duration of the spark. During this time the piston and counter-electrode perform a relative movement over a certain path of the counter-electrode. Therefore the moment at which ignition actually takes place can be varied within certain limits, with respect to the movement of the piston. The moment of ignition can therefore be adapted to the prevailing operating conditions.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.